1. Field of the Invention
The present invention relates to a storage element composed of a storage layer to store therein the magnetization state of a ferromagnetic layer as information and a magnetization fixed layer whose direction of magnetization is fixed, the direction of the magnetization of the storage layer being changed with application of an electric current and a memory including this storage element. More particularly, the present invention is suitable for use as the application to a nonvolatile memory.
2. Description of the Related Art
In information equipment such as a computer, a DRAM (dynamic random-access memory) operable at a high speed and which is high in density is widely used as a random-access memory (RAM).
However, since the DRAM is a volatile memory whose stored information is lost when operating power is removed, a nonvolatile memory whose information may not be lost even when operating power is removed has been desired.
Then, a magnetic random-access memory (MRAM) capable of recording information based on magnetization of a magnetic material receives a remarkable attention as a nominated nonvolatile memory and it is now under development (see Cited Non-Patent Reference 1, for example).
The MRAM is able to record information by inverting magnetization of a magnetic layer of a magnetic storage element located at an intersection point of address wirings based on electric current magnetic fields generated from address wirings with application of electric currents to two kinds of address wirings (word line and bit line) substantially perpendicular to each other.
FIG. 1 of the accompanying drawings is a schematic diagram (perspective view) showing an arrangement of a MRAM according to the related art.
As shown in FIG. 1, a drain region 108, a source region 107 and a gate electrode 101 constructing a selection transistor to select each memory cell are respectively formed on a semiconductor substrate 110 such as a silicon substrate at its portions isolated by an element isolation layer 102.
Also, a word line 105 that is extended in the front and back direction in FIG. 1 is provided over the gate electrode 101.
The drain region 108 is formed common to the right and left selection transistors in FIG. 1 and a wiring 109 is connected to this drain region 108.
A magnetic storage element 103 including a storage layer of which magnetization direction is inverted is located between the word line 105 and a bit line 106 located above the word line 105 and which is extended in the right and left direction in FIG. 1. This magnetic storage element 103 is composed of a magnetic tunnel junction device (MTJ device), for example.
Further, the magnetic storage element 103 is electrically connected to the source region 107 through a bypass line 111 of the horizontal direction and a contact layer 104 of the upper and lower direction.
When electric currents flow through the word line 105 and the bit line 106, respectively, an electric current magnetic field is applied to the magnetic storage element 103 to invert the magnetization direction of the storage layer of the magnetic storage element 103, thereby making it possible to record information in the storage layer.
Then, in the magnetic memory such as the MRAM, in order to stably hold recorded information, it is necessary that a magnetic layer (storage layer) to record information should have constant coercive force.
On the other hand, in order to rewrite recorded information, an electric current of a certain magnitude should flow through the address wiring.
However, as the element constructing the MRAM is microminiaturized increasingly, the address wiring also is decreased in thickness so that an electric current of a sufficient magnitude may not flow through such thin address wiring.
Magnetization inversion done by spin injection is that magnetization in other magnetic material may be inverted by injecting spin polarized electrons, which have passed the magnetic material, into other magnetic material.
For example, when an electric current flows through a giant magnetoresistive effect device (GMR device) or a magnetic tunnel junction device (MTJ device) in the direction perpendicular to the film plane, the magnetization direction of at least a part of the magnetic layer of these devices can be inverted.
The magnetization inversion based on spin injection has a merit in that, even when the element is microminiaturized, magnetic inversion can be realized by a small electric current.
FIGS. 2 and 3 are schematic diagrams showing a magnetic memory having an arrangement using the above-mentioned magnetization inversion based on spin injection. FIG. 2 is a perspective view showing a magnetic memory and FIG. 3 is a cross-sectional view thereof.
As illustrated, a drain region 58, a source region 57 and a gate electrode 51 constructing a selection transistor to select each memory cell are respectively formed on a semiconductor substrate 60 such as a silicon substrate at its portions isolated by an element isolation layer 52. Of these elements, the gate electrode 51 serves as a word line extended in the front and back direction in FIG. 2 as well.
The drain region 58 is formed common to right and left selection transistors in FIG. 2 and a wiring 59 is connected to this drain region 58.
A storage element 53 including a storage layer whose magnetization direction is inverted based on spin injection is located between the source region 57 and the bit line 56 extended in the right and left direction in FIG. 2.
This storage element 53 is composed of a magnetic tunnel junction element (MTJ device), for example. In FIG. 3, reference numerals 61 and 62 denote magnetic layers. Of the magnetic layers 61 and 62 of the two layers, one magnetic layer is used as a magnetization fixed layer of which magnetization direction is fixed and the other magnetic layer is used as a magnetization free layer of which magnetization direction may be changed, that is, storage layer.
Also, the storage element 53 is connected to the bit line 56 and the source region 57 through upper and lower contact layers 54. As a result, with application of an electric current to the magnetic memory element 53, it is possible to invert the magnetization direction of the storage layer based on spin injection.
The above-mentioned memory having the arrangement using magnetization inversion based on spin injection has features in that a device structure can be simplified as compared with the ordinary MRAM shown in FIG. 1.
Also, this memory has a merit in that, since the magnetization inversion based on spin injection is used, even when the element is microminiaturized increasingly, a write electric current can be prevented from being increased as compared with the ordinary MRAM which may invert the magnetization direction based on external magnetic fields.
In the above-mentioned memory having the arrangement to use the magnetization inversion based on the spin injection, in order to suppress power consumption more, it is necessary to decrease an inputted electric current by improving spin injection efficiency.
Also, in order to increase a magnitude of a read signal, it is necessary to maintain a large magnetoresistance variation. To this end, it is effective in making intermediate layers adjoining both sides of the storage layer become tunnel barrier layers.
In this case, it is inevitable that a withstand voltage of a barrier layer will be limited. From this point of view, it is necessary to suppress an electric current required upon spin injection.
Therefore, as a solution for suppressing an electric current required upon spin injection, there has been proposed an arrangement in which a storage element has a lamination layer structure of magnetization fixed layer/intermediate layer/storage layer/intermediate layer/magnetization fixed layer such that magnetization directions of magnetization fixed layers provided above and below a storage layer may be directed in the opposite directions (see Cited Patent Reference 2).
Then, the above-described Cited Patent Reference 2 discloses the fact that spin injection efficiency can be doubled by making the magnetization directions of the upper and lower magnetization fixed layers become opposite to each other.
[Cited Non-Patent Reference 1]: NIKKEI ELECTRONICS, 2001, Vol. 2. 12 (pp. 164 to 171)
[Cited Patent Reference 1]: Official Gazette of Japanese laid-open patent application No. 2003-17782
[Cited Patent Reference 2]: Specification of U.S. laid-open patent application No. 2004/0027853
Undoubtedly, from a theory standpoint, it is considered that spin injection efficiency can be doubled by the structure described in the above-described cited patent reference 2.
However, having manufactured a storage element having the structure described in the above-described cited patent reference 2 in actual practice and examined characteristics of this storage element, it is to be appreciated that satisfactory results could not be obtained theoretically so that satisfactory improvements of spin injection efficiency could not be recognized.